A typical integrated circuit comprises a plurality of electrically isolated active regions located on a common silicon semiconductor substrate. The active regions are generally formed by masking the substrate with patterned layers of silicon dioxide and silicon nitride to form the active regions under the patterned layers. The active regions are electrically isolated by oxidizing the exposed substrate surfaces to selectively form a thick field oxide, and then further processed to produce active components such as transistors.
In forming the active regions, a thin silicon dioxide layer is formed directly on the substrate and a thick silicon nitride layer is formed over the silicon dioxide layer. The silicon dioxide layer serves to buffer any mismatch in thermal expansion which occurs between the substrate and the silicon nitride layer during the substrate oxidation process, thereby minimizing crystallographic defects in the substrate. The thick layer of silicon nitride acts as a barrier to prevent oxidation of the active region during the oxidation of the exposed substrate surfaces.
The method described above is plagued by the passage of oxygen laterally through the thin layer of silicon dioxide and into the active region, resulting in "bird's beak" formations of silicon dioxide along the edges of the active regions. These "bird's beak" formations alter the shape and area of the active regions, reducing the usable area of the substrate.
Ghezzo U.S. Pat. No. 4,333,964, assigned to the instant assignee, shows a method of minimizing "bird's beak" formations by using a multilayer structure including a very thin layer of silicon nitride in direct contact with the substrate to inhibit the oxidation of the active regions near the edges from the lateral flow of oxygen. The use of the additional thin nitride layer, however, reduces, but does not eliminate completely the crystallographic defects resulting from the thermal expansion mismatch described above.
Chow and Ghezzo U.S. Pat. No. 4,333,965, assigned to the instant assignee, shows another method of minimizing "bird's beak" formations by forming oxygen masking spacers of silicon nitride over the sidewalls of the active region mesa, the spacers inhibiting lateral oxidation of the mesa. The method taught by this patent, however, has the disadvantage of producing spacers too thin to entirely block the lateral oxidation. Moreover, this method requires substantial etching of the substrate to form mesas in the active region, thereby complicating the fabrication process.